Successful restoration of blurred images depends primarily on the knowledge about the degradation parameter.Defocus blur model in the frequency domain is characterized by concentric rings and the blur radius of the point spread function (PSF) can be identified conveniently in the frequency field for people by manual means rather than for computer.This paper introduces a practical method for computer to estimate the defocus blur parameter in cepstrum area.Fourier transform plays an intermediate role in the path to cepstrum domain.We suggest a weighted adjustment operation in the frequency domain and then convert it to the cepstrum field to increase the accuracy of recognition.